652 THE POPULAR SCIENCE MONTHLY. 



the skin. This is the physiology of blushing. Congestion is produced 

 by a permanent expansion of the capillaries. We might call blushing 

 a momentary congestion. 



In an emergency the arteries are capable of great exj^ansion. They 

 are connected by branches, or loops ; and, in case of stoppage of the 

 circulation in a large artery, either by disease or a surgical operation, 

 another will, after a time, perhaps a few hours, expand to a size requi- 

 site to carry sufficient blood. This variation in the carrying capacity 

 of the arteries is the imjDortant secondary means of adapting the 

 amount of blood to the wants of any part of the body. 



In man there is a greater and more direct supply of blood to the 

 right arm, corresponding to the greater use of that limb. But, in 

 birds, equality of supply is necessary for the equality of strength 

 needed in steady flight. 



For protection, the arteries are as deep-seated as possible, lying 

 beneath the , muscles, and appearing rarely at the surface. At the 

 joints they form loops, so that the circulation may not be stopped by 

 compression of a single trunk. A fine example of adaptation is seen 

 in the arm of the lion, where the main artery, to be protected by the 

 powerful muscles, passes through a perforation in the bone. 



- 



ABOUT MEASURES OF LET^GTH. 



By EOMYN HITCHCOCK. 



FEW realize the great practical importance of extreme accuracy 

 in standards of weight and extension, and it is not generally 

 known what degree of accuracy has been attained in the measure- 

 ment of the standards of length now in use in diiferent countries. 

 The carpenter's foot-rule and the tailor's yard are familiar articles, 

 but, if asked, probably neither the carpenter nor the tailor could 

 tell whether there is any means by which the true length of a foot 

 or a yard can be determined. It is clear, however, that there must 

 be a standard with which the common measures should be made to 

 agree, in order to have the same absolute value. But we may reflect 

 that the constant use of any measure will change its length, and that 

 it will eventually become worn out. We can, then, readily under- 

 stand the great value of an accepted standard, from which copies can 

 be made, thus preventing any gradual alteration in our measures. 

 Such standards of reference are properly held in the custody of 

 national governments, scientific societies, and institutions. 



It is by no means a simple process to compare one measure with 

 another, and to determine the variation between the tw^o. On the 

 contrary, the utmost skill and long experience are required for such 



